(1) Field of the Invention
The present invention relates to the field of rotary-wing aircraft, and more particularly to control systems for directional piloting in yaw of a rotorcraft, in particular of a helicopter.
The invention provides a yaw flight control system for a rotorcraft that makes use of at least one member of the type operated by a human, in particular a human pilot of the rotorcraft, and of calculation means that operate actuators for acting on an anti-torque rotor of the rotorcraft.
(2) Description of Related Art
Rotorcraft are fitted with rotors, each constituting a rotary wing made up of a plurality of blades, which blades extend radially around a rotary drive axis of the rotary wing. A main rotor having an axis that is substantially vertical relative to the rotorcraft provides the rotorcraft with lift and possibly also with propulsion, and/or enables it to be maneuvered in various directions, in pitching, in roll, and in elevation, in particular. An anti-torque device serves to maneuver the rotorcraft in turning about a yaw axis. Such an anti-torque device is constituted in particular by a rotor having its axis substantially horizontal relative to the rotorcraft. Such a substantially horizontal axis rotor is commonly formed by a tail rotor, or else by a propulsive propeller for a rotorcraft presenting high forward speed and a long range.
The rotorcraft has a plurality of movable control members of the type operated by a human in order to pilot the rotorcraft. The movable control members generate flight controls depending on how they are operated by a human pilot of the rotorcraft, thereby acting on the blades of the rotors in order to modify their pitch, cyclically or collectively.
The movable control members used for acting on the main rotor are conventionally arranged as a member that is operated by the hand of the human pilot of the rotorcraft, such as a stick or a lever hinged to the floor of the rotorcraft, or indeed a “joystick” type stick installed in an armrest of a seat on which the human pilot is sitting.
A movable member for controlling cyclic pitch enables the human pilot to modify the angle of attack of the blades of the main rotor in differential manner, thereby varying the orientation of the rotorcraft in pitching and in roll. A movable member for controlling the collective pitch of the blades of the main rotor enables the human pilot to modify the pitch of all of the blades together, and consequently to vary the lift of the rotary wing, so as to maneuver the rotorcraft in elevation.
The movable control member for enabling the human pilot to act on the anti-torque rotor is traditionally arranged as a rudder bar including a set of pedals. The rudder bar enable the human pilot to vary the angle of attack of all of the blades of the anti-torque rotor together, thereby varying the thrust that it generates at the tail of the rotorcraft. Thrust from the anti-torque rotor thus serves either to perform a yaw maneuver, or else to compensate the yaw torque that is generated by the main rotor, with any variation in the propulsion of the rotorcraft from the main rotor involving concurrent action on of the anti-torque rotor in order to adjust the effect that it produces correspondingly.
In particular embodiments of the movable member for controlling the anti-torque rotor, the rudder bar is replaced by a stick or a joystick hinged to the floor of the rotorcraft or to a pilot's seat.
For example, according to FR 2 918 348 (the State of France), the controls normally operable by the human pilot using the rudder bar is transferred to a stick in order to enable a paraplegic human pilot to control the rotorcraft in yaw without using his or her legs.
Also by way of example, according to U.S. Pat. No. 5,395,077 (Wolford T.), the rudder bar is eliminated and replaced by a control joystick.
Conventionally, movable control members are connected to the rotors via a corresponding drive channel. Such a drive channel comprises in particular a remote mechanical transmission mechanism that is interposed between the movable control member operable by the human pilot and at least one rotor to which the drive channel is allocated. Such remote mechanical transmission mechanisms are commonly made up of connecting rods, links, cables, and/or other analogous remote mechanical transmission members.
The drive channel is preferably fitted with a servo-control that provides accurate control over the operation of the rotors and/or that serves to amplify the power of the flight commands that are transmitted via the drive channel. A flight command transmitted to that servo-control is logically compared with the state of the blades of the rotor in order to detect an error, and the difference value obtained by the comparison is amplified and used to correct the detected error.
In order to improve the comfort of the human pilot, movable control members have been proposed that generate electrical signals that are processed by an intermediate computer in order to operate the servo-controls forming part of the drive channel. A flight command issued by the human pilot by operating the movable control member is not transmitted mechanically to the corresponding rotor via the remote mechanical transmission mechanism allocated thereto, but instead generates electrical signals. These electrical signals are taken into account by the intermediate computer, which then transmits activation commands to a servo-control in order to operate it. Operation of the servo-control causes the pitch of the blades to be varied by means of a mechanical transmission mechanism.
It is also common for rotorcraft to be fitted with an autopilot that, where necessary, can act in substitution for the movable control members of the type operated by a human. The autopilot is activated on the basis of an execution order issued by a human pilot by means of a movable control member of the type operated by a human, such as a control button or an analogous control member.
In a function of keeping to a reference, the autopilot operates the drive channel as a function of a reference flight setpoint. The reference flight setpoint is previously issued by the human pilot or is determined by the autopilot on the basis of observation information about a state of progression of the rotorcraft. Activating the autopilot causes the progression of the rotorcraft to be maintained relative to observation information evaluated at the time of that activation.
More particularly, the autopilot makes use of calculation means that generate orders automatically for using actuators allocated to operating the rotors. The autopilot comprises computers that implement piloting relationships for operating the rotors in order to maintain controlled progression of the rotorcraft in the absence of the human pilot issuing flight commands via movable control members of the type operated by a human.
In the reference-keeping function provided by the autopilot, a state of progression of the rotorcraft is maintained by the autopilot acting on the use of the actuators by making a comparison between the reference flight setpoints and the actual state of progression of the rotorcraft. The reference flight setpoint is unchanging information to which the autopilot refers in order continuously to correct the state of progression of the rotorcraft. Such a correction is controlled by the autopilot as a function of the actual state of progression of the rotorcraft as established on the basis of said observation information delivered by various measurement and calculation means with which the rotorcraft is fitted.
The actuators are commonly electrically powered or preferably hydraulically powered since hydraulic power is considered to be more reactive. The actuators are commonly referred to respectively as a “trim” actuator and as a “series” actuator.
The trim actuator is a bidirectional member that is placed in the drive channel in parallel with the rudder bar in order to transmit a flight command to the rotor via the drive channel. The trim actuator serves to operate the drive channel in substitution for the movable control member. The series actuator is a bidirectional member that is placed in series in the drive channel and that is used in particular for stabilizing the operation of the rotor.
The autopilot generates activation commands for activating the series actuator in order to act on the behavior of the rotor. The trim actuator is suitable for being activated in order to recenter the series actuator and in order to remedy the reduced authority of the series actuator. Various automatic flight control system architectures have been proposed.
For example, according to EP 0 296 951 (Aerospatiale), information from a plurality of movable control members of an aircraft is combined and processed by a computer in order to provide joint control of roll and yaw flight maneuvers.
Document FR 2 339 920 (Sperry Rand Corp.) describes an aircraft control system incorporating a system for increasing the stability of the aircraft and for performing yaw flight maneuvers.
More particularly, documents WO 93/05460 (United Tech. Corp.) and WO 93/05460 (United Tech. Corp.) describe rotorcraft flight control systems including means for automatically adapting flight commands in order to maneuver the rotorcraft in yaw.
The technological background close to the present invention can also be discovered by reference to documents U.S. Pat. No. 3,528,633 (Knemeyer S.), U.S. Pat. No. 4,392,203 (Fischer W. C. et al.), and U.S. Pat. No. 4,527,242 (McElreath K. W.) which describe various rotorcraft yaw flight control devices having calculation means incorporated in an autopilot of a rotorcraft.
More specifically, in the field of aviation, objective-type flight control systems are known. Movable flight control members operated by a human pilot are used to generate electrical signals that correspond to objective-type flight commands relating to a state of progression that the airplane is to achieve. Objective-type flight commands are transmitted continuously to an objective computer that determines how to use actuators so as to modify the state of progression of the aircraft. The objective computer generally controls orders for activating actuators by comparing the objective-type flight controls issued by the human pilot and observation information about an actual state of progression of the aircraft.
For rotorcraft, it is sometimes found in practice that there are drawbacks relating to yaw piloting via the rudder bar.
For example, in manual piloting mode, for a position of the rudder bar determined by the human pilot, the turning rate imparted to the rotorcraft about the yaw axis is not constant because of the influence of the external environment and in particular because of wind conditions. Consequently, the human pilot needs to adapt variations in the yaw maneuvering of the rotorcraft dynamically and to correct the turning of the rotorcraft depending on the rate of turn that is actually obtained.
As another example, in manual piloting mode, the entire drive channel connecting the rudder bar to the anti-torque rotor is made up of mechanical members that are subject to friction. Clearances and inaccuracies in the movement of the moving mechanical members making up the mechanical drive channel are inevitable, to the detriment of the “feel” perceived by the human pilot and to the detriment of the pertinence of the flight commands transmitted to the anti-torque rotor.
It is also observed in automatic piloting mode that the human pilot may act unintentionally on the rudder bar, e.g. because the pilot may take up a poor posture in the cockpit and/or because it is difficult for the human pilot to sense small movements of the rudder bar through feet wearing boots used by the human pilot to move the pedals of the rudder bar. The human pilot may unintentionally oppose the action of the actuators on the drive channel, such as movement of the trim actuator used for recentering the series actuator. The series actuator may become saturated, thereby leading to a loss of stabilization of the rotorcraft.
In an automatic piloting configuration of the rotorcraft, action by the human pilot on the rudder bar leads to the reference-keeping function of the autopilot being interrupted so as to allow the human pilot to act directly on the rotorcraft. The reference-keeping function performed by the autopilot can thus be stopped unintentionally by the human pilot, and without the human pilot being immediately aware that this has happened.